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Florence's floodwaters breached a dam holding back a large reservoir at a Wilmington power plant Friday, and coal ash from an adjacent dump could be flowing into the nearby Cape Fear River.

Duke Energy spokeswoman Paige Sheehan said the company does not believe the breach at the L.V. Sutton Power Station poses a significant threat for increased flooding to nearby communities because the river is already running high after the hurricane.

Floodwaters from the Cape Fear opened several breaches overnight in the earthen dam at Sutton Lake, a 1,100-acre (445-hectare) reservoir at the power plant. Water from the lake then flooded one of three large coal ash dumps lining the lakeshore.

Sheehan said the company can't rule out that ash might be escaping from the flooded dump and washing into the river.

Grey material that the company characterized as "coal combustion byproducts" could be seen floating in both the lake and river Friday.

The ash left over when coal is burned to generate electricity contains an array of components, including mercury, lead, arsenic and other toxic heavy metals. Duke said Friday that the inundated basin at the plant contains about 400,000 cubic yards (305,820 cubic meters) of ash.

Floodwaters at the site were continuing to rise Friday. The area received more than 30 inches (75 centimetres) of rain from former Hurricane Florence, with the Cape Fear River expected to crest Saturday.

North Carolina's top environmental regulator said the extent of the potential environmental harm is not yet known.

"What we don't know at this point is if any coal ash has filtered into the Cape Fear River," said Mike Regan, secretary for the state Department of Environmental Quality. "We plan to conduct flyovers ... to see if we can ascertain that."

Security personnel for Duke blocked access Friday to Sutton Lake Road, which leads to a public dock on the reservoir, a popular local destination for boating and fishing.

Duke denied a request for an Associated Press reporter at the scene Friday to pass the barricade, saying that the situation at the lake "continues to change" and is "not safe." Aerial photos released by the company showed a wide breach in the earthen dam and the affected ash dump largely underwater.